Overview of Vitamins
Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe upper limit (tolerable upper intake level) has been determined for some vitamins. Intake above this limit increases the risk of a harmful effect (toxicity).
Consuming too little of a vitamin can cause a nutritional disorder. However, people who eat a variety of foods are unlikely to develop most vitamin deficiencies. Deficiency of vitamin D is an exception. Vitamin D deficiency is common among certain groups of people (such as older people) even if they eat a variety of foods. For other vitamins, a deficiency can develop if people follow a restrictive diet that does not contain enough of a particular vitamin. For example, vegans, who consume no animal products, may become deficient in vitamin B12, which is available in animal products. Deficiency of the B vitamins biotin or pantothenic acid almost never occurs.
Consuming large amounts (megadoses) of certain vitamins (usually as supplements) without medical supervision may also have harmful effects.
Vitamins are called essential micronutrients because the body requires them but only in small amounts.
The body does not store most vitamins. Deficiencies of these vitamins usually develop in weeks to months. Therefore, people must consume them regularly.
Vitamins A, B12, and D are stored in significant amounts, mainly in the liver. Vitamins A and D are also stored in fat cells. Deficiencies of these vitamins take more than a year to develop.
Because many people eat irregularly or do not eat a variety of foods, they may not get enough of some vitamins from foods alone. If they do not get enough, the risk of certain cancers or other disorders may be increased. People may then take a multivitamin. However, for most people, taking multivitamins does not appear to reduce risk of developing cancer or heart or blood vessel (cardiovascular) disorders.
Vitamins
Vitamin |
Good Sources |
Main Functions |
Recommended Dietary Allowance for Adults |
Safe Upper Limit |
|
Biotin |
Liver, kidneys, meats, eggs, milk, fish, dried yeast, sweet potatoes, seeds, and nuts |
Required for the processing (metabolism) of carbohydrates and fatty acids |
30 micrograms (but no RDA has been established) 35 micrograms for breastfeeding women |
— |
|
Folate (folic acid) |
Raw green leafy vegetables, asparagus, broccoli, fruits (especially citrus), liver, other organ meats, dried yeast, and enriched breads, pastas, and cereals (Note: Extensive cooking destroys 50–95% of the folate in food.) |
Required for the formation of red blood cells, for DNA and RNA synthesis, and for normal development of the nervous system in a fetus |
400 micrograms 600 micrograms for pregnant women 500 micrograms for breastfeeding women |
1,000 micrograms |
|
Niacin (nicotinic acid or nicotinamide) |
Dried yeast, liver, red meat, poultry, fish, legumes, and whole-grain or enriched cereal products and bread |
Required for the metabolism of carbohydrates, fats, and many other substances and for the normal functioning of cells |
14 milligrams for women 16 milligrams for men 18 milligrams for pregnant women 17 milligrams for breastfeeding women |
35 milligrams |
|
Pantothenic acid |
Liver, beef, egg yolks, yeast, potatoes, broccoli, and whole grains |
Required for the metabolism of carbohydrates and fats |
5 milligrams (but no RDA has been established) 6 milligrams for pregnant women 7 milligrams for breastfeeding women |
— |
|
Riboflavin (vitamin B2) |
Milk, cheese, liver, meat, fish, eggs, and enriched cereals |
Required for the metabolism of carbohydrates and proteins and for healthy mucous membranes, such as those lining the mouth |
1.1 milligrams for women 1.3 milligrams for men 1.4 milligrams for pregnant women 1.6 milligrams for breastfeeding women |
— |
|
Thiamin (vitamin B1) |
Dried yeast, whole grains, meat (especially pork and liver), enriched cereals, nuts, legumes, and potatoes |
Required for the metabolism of carbohydrates, proteins, and fats and for normal nerve and heart function |
1.1 milligrams for women 1.2 milligrams for men 1.4 milligrams for pregnant or breastfeeding women |
— |
|
Vitamin A (retinol) |
As vitamin A: Fish liver oils, liver, egg yolks, butter, cream, and fortified milk As carotenoids (converted to vitamin A in the body), such as beta-carotene: Dark green, yellow, and orange vegetables and yellow and orange fruits |
Required to form light-sensitive nerve cells (photoreceptors) in the retina, helping maintain night vision Helps maintain the health of the skin, cornea, and lining of the lungs, intestine, and urinary tract Helps protect against infections |
700 micrograms for women 900 micrograms for men 770 micrograms for pregnant women 1,300 micrograms for breastfeeding women |
3,000 micrograms |
|
Vitamin B6 (pyridoxine) |
Dried yeast, liver, other organ meats, whole-grain cereals, fish, and legumes |
Required for the metabolism of carbohydrates, amino acids, and fats, for normal nerve function, for the formation of red blood cells, and for healthy skin |
1.3 milligrams for younger women and men 1.5 milligrams for women older than 50 1.7 milligrams for men older than 50 1.9 milligrams for pregnant women 2.0 milligrams for breastfeeding women |
100 milligrams |
|
Vitamin B12 (cobalamins) |
Meats (especially beef, pork, liver, and other organ meats), eggs, fortified cereals, milk, clams, oysters, salmon, and tuna |
Required for the formation and maturation of red blood cells, for nerve function, and for DNA synthesis |
2.4 micrograms 2.6 micrograms for pregnant women 2.8 micrograms for breastfeeding women |
— |
|
Vitamin C (ascorbic acid) |
Citrus fruits, tomatoes, potatoes, broccoli, strawberries, and sweet peppers |
Required for the formation, growth, and repair of bone, skin, and connective tissue; for healing of wounds and burns; and for normal function of blood vessels Acts as an antioxidant, protecting cells against damage by free radicals Helps the body absorb iron |
75 milligrams for women 90 milligrams for men 85 milligrams for pregnant women 120 milligrams for breastfeeding women 35 milligrams more for smokers |
2,000 milligrams |
|
Formed in the skin when the skin is exposed to direct sunlight Fortified milk and dairy products, fatty fish, fish liver oils, liver, and egg yolks |
Promotes the absorption of calcium and phosphorus from the intestine Required for bone formation, growth, and repair Strengthens the immune system and reduces the risk of autoimmune disorders |
600 units for people aged 1‒70 800 units for people older than 70 |
4,000 units |
|
|
Vegetable oil, nuts, seeds, green leafy vegetables, and wheat germ |
Acts as an antioxidant, protecting cells against damage by free radicals |
15 milligrams (22 units of natural or 33 units of synthetic) 19 milligrams for breastfeeding women |
1,000 milligrams |
|
|
Green leafy vegetables (such as collards, spinach, and kale) and soybean and canola oils |
Helps in the formation of blood clotting factors and thus is necessary for normal blood clotting Required for healthy bones and other tissues |
90 micrograms for women 120 micrograms for men |
— |
|
|
DNA = deoxyribonucleic acid; RNA = ribonucleic acid; RDA=recommended daily allowance. |
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Some vitamins are fat soluble. Other vitamins are water soluble. The difference between fat and water soluble affects nutrition in several ways.
Fat-soluble vitamins
Fat-soluble vitamins dissolve in fats (lipids) and include
Fat-soluble vitamins are stored in the liver and in fatty tissues. If too much of the fat-soluble vitamins A or D are consumed, they can accumulate and may have harmful effects.
Because fats in foods help the body absorb fat-soluble vitamins, a low-fat diet may result in a deficiency. Some disorders interfere with absorption of fats and thus of fat-soluble vitamins. These are called malabsorption disorders. Examples are chronic diarrhea, Crohn disease, cystic fibrosis, certain pancreatic disorders, and blockage of the bile ducts. Some drugs, such as mineral oil, have the same effect. Fat-soluble vitamins dissolve in mineral oil, which the body does not absorb. So when people take mineral oil (for example, as a laxative), it carries these vitamins unabsorbed out of the body.
Cooking does not destroy fat-soluble vitamins.
Water-soluble vitamins
Water-soluble vitamins dissolve in water and include
B vitamins include biotin, folate (folic acid), niacin, pantothenic acid, riboflavin (vitamin B2), thiamin (vitamin B1), vitamin B6 (pyridoxine), and vitamin B12 (cobalamins).
Water-soluble vitamins are eliminated in urine and tend to be eliminated from the body more quickly than fat-soluble vitamins. Water-soluble vitamins are more likely to be destroyed when food is stored and prepared. The following can help prevent the loss of these vitamins:
Causes
Disorders that impair the intestine’s absorption of food (called malabsorption disorders) can cause vitamin deficiencies.
Some disorders impair the absorption of fats. These disorders can reduce the absorption of fat-soluble vitamins—A, D, E, and K—and increase the risk of a deficiency. Such disorders include chronic diarrhea, Crohn disease, cystic fibrosis, certain pancreatic disorders, and blockage of the bile ducts.
Some types of weight-loss (bariatric) surgery can also interfere with absorption of vitamins.
Liver disorders and alcoholism can interfere with the processing (metabolism) or storage of vitamins.
In a few people, hereditary disorders impair the way the body handles vitamins and thus cause a deficiency.
If people must be fed intravenously for a long time or if the formula used lacks the needed nutrients, people may develop a vitamin (or mineral) deficiency.
Drugs can also contribute to deficiency of a vitamin. They may interfere with absorption, metabolism, or storage of a vitamin.
Some Drugs That Cause Vitamin Deficiency
Drugs Mentioned In This Article
| Generic Name | Select Brand Names |
|---|---|
Sulfasalazine |
AZULFIDINE |
phenobarbital |
No US brand name |
Penicillamine |
CUPRIMINE |
tetracycline |
ACHROMYCIN V |
methotrexate |
OTREXUP |
Cycloserine |
SEROMYCIN |
Triamterene |
DYRENIUM |
imipramine |
TOFRANIL |
isoniazid |
LANIAZID |
Metformin |
GLUCOPHAGE |
phenytoin |
DILANTIN |
Primidone |
MYSOLINE |
warfarin |
COUMADIN |
Levodopa |
Levodopa |
Rifampin |
RIFADIN, RIMACTANE |
niacin |
NIACOR, NIASPAN |
